A cementing operation often is used to plug the bore of a well that is to be abandoned for any reason, for example where well testing has shown that hydrocarbons are not present in commercial quantities (or not present at all). Similar procedures are used where a well is to be "kicked-off" to change its direction at some point along its path. Typically a pipe string is run into the well bore, and cement slurry is pumped down the pipe. The slurry displaces drilling mud or other fluids in the pipe and passes out the lower end thereof to displace the fluid standing in the annular space between the outer wall of the pipe string and the wall of the borehole. At present, the cement slurry is forced down the pipe string by a following displacement fluid with or without a plug that is used to separate the cement slurry and the displacement fluid. After the cement slurry has been pumped into the annulus, the drill pipe is withdrawn, leaving the slurry at the bottom of the well to set up and harden to form a solid plug.
Ideally, just sufficient cement slurry is pumped into the pipe string to fill the annular space to the required height in a manner such that virtually no slurry is left in the bottom of the inside of the pipe string. This procedure is carried out by placing in the pipe string, above a calculated quantity of cement slurry, a fluid displacement plug which is driven along the pipe string by the application of pressure on top of the plug. The displacement fluid may be a typical substance well known to those skilled in the art. Thus the cement slurry is driven along in front of the plug, as the displacement fluid, behind the plug, is pumped under pressure.
Considerable skill has been required during the cement plugging operation to know precisely when to stop the pumps so that the plug is just reaching the bottom of the pipe string. If pumping is continued after the plug reaches bottom, displacement fluid will be pumped beyond the end of the pipe string and displace the cement slurry from its proper position around the outside of the string. This can result in damage to the cement mix so as to prevent proper setting thereof. Such an occurrence is known as over-displacement. If the converse occurs, and the pumping is discontinued before the plug reaches the bottom, the cement may set in the pipe string and will have to be drilled out at a later stage. Clearly such extra drilling is time consuming and expensive.
Additionally, when the pumps are turned off before withdrawing the pipe string, the weight of the column of cement slurry around the outside of the string may be greater than the weight of column of the displacement fluid inside the string. This situation can cause the cement slurry to flow back down outside the pipe and up into the hollow interior thereof. This effect, known as the "U-tube" effect, may result in cement slurry remaining in the pipe string after its withdrawal, with resulting difficulties as will be apparent.
Known devices have provided a tubular structure that is connected in the pipe string to present an obstruction in the bore thereof. The obstruction may have a shape such that a fluid displacement plug can move past the obstruction under the force of the displacement fluid behind the plug, but which prevents return of the plug back up the pipe. In accordance with the present invention, a restriction is placed at a selected location in the pipe string so that a surge of pressure may be detected at the surface to indicate the precise position of the plug so that pumping can be terminated or reduced immediately upon detection of the pressure surge.
Plug catchers are known that function to trap the cement plug in the lower end of the pipe string. For example a catcher offered by the Halliburton Company, Duncan, Okla., is a latch-down, indicating type plug catcher. In operation, the catcher is placed near the end of the string in the drill pipe and lowered to the desired depth. When the operator is ready to start pumping the cement slurry, a bottom plug placed in the pipe string is started, and the desired amount of cement slurry is pumped behind this plug. The plug will pass through the catcher and out the bottom of the drill pipe. A top plug that is retained in a plug container during displacement of the cement then is released and pumped into the pipe behind the cement slurry. When the plug reaches the catcher, the plug enters a baffle and is locked and sealed in place.
The operator must take precautions at this time so that the differential pressure across the top plug does not exceed a maximum value which would shear the baffle which is held by shear screws. The pipe is then lifted upward until its lower end is at a desired location on top of the cement slurry, and pressure is supplied in the pipe so as to shear the screws and release the baffle. The baffle and plug will then move to the bottom end of the catcher, uncovering ports. The baffle and plug are locked in the lower end of the tool and will remain throughout a reverse circulation process.
After this single plug operation has been performed, the drill pipe must be tripped out of the hole so that the plug catcher can be recovered and disassembled to retrieve the top plug and baffle.
The above-mentioned system has a number of disadvantages which makes its use undesirable. One disadvantage is that the pipe string must be tripped after each plug job so as to retrieve the top plug and baffle in the catcher. Another disadvantage is that it is necessary to disassemble the plug catcher upon its retrieval to replace worn or damaged seals and lock rings. The baffle must then be reassembled in the catcher with new shear screws. Additionally, the ports in the catcher are positioned perpendicular to the axis thereof so as to direct the flow of cement slurry directly against the formation wall. Another disadvantage is that the plug catcher does not provide a means for pumping away the plug member caught therein, if an emergency requiring such an action arises.
A more detailed discussion of cementing heads, plug containers and plug launching subs may be found in the following: U.S. Pat. Nos. 2,071,392, 4,047,566 and 4,164,980. U.S. Pat. No. 3,777,819 discloses a plug container having cement plug valves for driving plug members therethrough.
One object of the present invention is to provide new and improved methods and apparatus for use in placing cement plugs.
Another object of this invention is to provide a new and improved plug catcher adapted for use in a well to catch and contain plug members used in cement plugging operations.
Another object of this invention is to provide a plug catcher of the type described which prevents pressure locking in the plug catcher and assures that the tubular member stays free of cement slurry.
Another object of this invention is to provide a new and improved plug catcher which is assembled from short length component parts to aid in the maintenance, handling and transporting thereof.
Still another object of this invention is to provide a new and improved plug catcher capable of catching and containing a plurality of plug members so as to save rig time and expense by eliminating tripping of the pipe string between plugging operations.
Another object of the present invention is to provide a new and improved cementing plug launching apparatus which comprises no moving parts and which is simple and reliable in operation.